Fiber-reinforced composite material is known, which is reinforced by combining fiber with resin. The fiber-reinforced composite material is exemplified by CFRP (Carbon Fiber reinforced Plastics). Because such fiber-reinforced composite material has a great advantageous effect of light-weight, it is applied to many products such as an aircraft structural member. The fiber-reinforced composite material is manufactured by VaRTM (Vacuum assisted Resin Transfer Molding) and RFI (Resin Film Infusion), in which laminated reinforced fiber is impregnated in resin and then the resin is hardened. In the fiber-reinforced composite material, defects such as a void and failure of impregnation of resin are sometimes caused. It is desired that such defects should be detected before the resin is hardened.
Japanese Patent No. 3,864,180 discloses an ultrasonic testing method which can clearly detect a defect portion by a noncontact reflection method. In the ultrasonic testing method, an ultrasonic wave is transmitted from a probe provided on one side of a test object and a reflected wave is received by the probe. The probe has a transmitter and a receiver. The ultrasonic wave is transmitted and received through an air layer between the transmitter and the receiver, and the test object. Relative positions of the transmitter, the receiver, and the test object are set so that an aerial transmission time between the transmitter and the receiver is longer than a transmission time of the reflected wave.
US Patent Publication US 2002/0088281A discloses an inspection method for performing inspection with use of a Lamb wave.